Pressure-induced quantum critical point in the heavily hydrogen-doped iron-based superconductor LaFeAsO

An iron-based superconductor ${\mathrm{LaFeAsO}}_{1\ensuremath{-}x}{\mathrm{H}}_{x}\phantom{\rule{0.16em}{0ex}}(0\ensuremath{\le}x\ensuremath{\le}0.6)$ undergoes two antiferromagnetic (AF) phases upon H doping. We investigated the second AF phase ($x=0.6$) using NMR techniques under pressure. At pressures up to 2 GPa, the ground state is a spin-density-wave state with a large gap; however, the gap closes at 4.0 GPa, suggesting a pressure-induced quantum critical point. Interestingly, the gapped excitation coexists with gapless magnetic fluctuations at pressures between 2 and 4 GPa. This coexistence is attributable to the lift up of the ${d}_{xy}$ orbital to the Fermi level, a Lifshitz transition under pressure.